1. Field of the Invention
This invention relates to hydraulic vane type pumps and more particularly to such pumps having undervane pressure to assist vane extension.
2. Description of the Prior Art
The known vane type pumps, as normal, have a volumetric efficiency up to 80%-90% when middle viscosity fluid (17-38 cst 50.degree. C.) is used. But the volumetric efficiencies are very low, when a low viscosity fluid (below 6 cst 50.degree. C.) is applied e.g. with high water base fluids (HWBF) (95% water, 5% additives, viscosity 1.1 cst) the volumetric efficiencies may go down as low as 30%-50% (working pressure 63 kgf/cm.sup.2 and displacement 10-32 ml/r).
According to the following reference:
(1) Retrofitted vane type pump handles high water base fluids "Design News" Vol. 37. No. 12.